Found by a two-stage search. A randomized fixed-cardinality local search (seed 1204) first produced a 21-point candidate with one nontrivial ordered solution after 891,781 moves. An exhaustive search of its five-replacement neighborhood then found this valid set after examining 6,024 valid 16-point bases and 86,488,865 legal-extension checks. Independently checked by literal enumeration of all 21^4 ordered quadruples: exactly 21 solutions, all diagonal.
David Renshaw · 2026-08-05 04:22:14 UTC